1. Field of the Invention
The present invention relates to a direct-injection internal combustion engine and a method of controlling the direct-injection internal combustion engine. More specifically, the present invention relates to a direct-injection internal combustion engine and a method of controlling the direct-injection internal combustion engine that inject the fuel via a fuel injection valve during a compression stroke or an expansion stroke while the engine is operating in a cold-start state.
2. Description of the Related Art
In recent years, direct-injection internal combustion engines are currently available, in which fuel is directly injected into the combustion chambers of a plurality of cylinders via fuel injection valves. In the direct-injection internal combustion engine, the combustion mode changes from a stratified-charge combustion to a homogeneous-charge combustion, according to the state in which the internal combustion engine is operating. The stratified-charge combustion is performed mainly during light load and low speed operation, such as when the engine is just started. When stratified-charge combustion is performed, fuel is injected into the combustion chamber via the fuel injection valve during at least one of the compression stroke and the expansion stroke. With regard to a wall-guided direct-injection internal combustion engine, fuel injected while the stratified-charge combustion is performed goes toward an ignition plug, running along a wall surface of the cylinder or the piston crown portion. When going toward the ignition plug, the fuel mixes with the intake air already introduced from the air intake path into the combustion chamber. The air-fuel mixture is ignited by the ignition of the ignition plug, which results in the combustion of the fuel in the mixture.
With regard to the wall-guided direct-injection internal combustion engine, because the temperature in the combustion chamber (i.e., the in-cylinder temperature) is low when the engine is just started, particularly in a cold-start state, part of the fuel injected during the stratified-charge combustion adheres the wall surface of the cylinder or the piston crown portion in a liquid state. Because the adhered fuel is in a liquid state, it is difficult to vaporize. Accordingly, even when the mixture is ignited, and the fuel is burned, most of the adhered fuel is not vaporized, and remains adhered in a liquid state. For this reason, there is a possibility that a large amount of smoke occurs in the combustion chamber.
As described in Japanese Patent Application Publication No. JP-A-2004-340040, a direct-injection internal combustion engine has been proposed that generates a vertical vortex (tumble flow), and maintains the vertical vortex until a later period of the compression stroke. The described engine also includes a fuel injection valve that has a penetration on the ignition plug side longer than the penetration on the piston side. The direct-injection internal combustion engine as described in Japanese Patent Application Publication No. JP-A-2004-340040 can reduce the smoke that occurs in the combustion chamber mainly by keeping the fuel from adhering the piston crown portion.
On the other hand, spray-guided direct-injection internal combustion engines have been proposed. In a spray-guided direct-injection internal combustion engine, the fuel injected during the stratified-charge combustion is mixed with the intake air introduced from the air intake path into the combustion chamber to form a mixture near the ignition plug. Thereafter, the mixture is ignited by the ignition of the ignition plug, which results in the combustion of the fuel in the mixture. In the case of the spray-guided direct-injection internal combustion engine, the fuel injected into the combustion chamber via the fuel injection valve is not directed to the ignition plug by causing the fuel to run along the wall surface of the cylinder or the piston crown portion. Instead, the fuel is directed to the ignition plug by the flow of the intake air in the combustion chamber. Thus, a spray-guided direct-injection internal combustion engine further reduces the smoke that occurs in the combustion chamber by greatly inhibiting the fuel from adhering the wall surface of the cylinder or the piston crown portion.
In a cold-start state, fuel is difficult to vaporize because the temperature in the combustion chamber is low. Especially in a case where the stratified-charge combustion is performed, because the air-fuel mixture is generally rich. The air-fuel mixture near the ignition plug, may include an over-rich region is generated locally in the combustion chamber. In the over-rich region, liquid fuel, or droplet fuel, is suspended. Accordingly, also in the case of the spray-guided direct-injection internal combustion engine, there is a possibility that, smoke occurs in the local over-rich region in the combustion chamber, although the amount thereof is less than that of the wall-guided direct-injection internal combustion engine.